This invention relates generally to high temperature, secondary electrochemical cells and batteries of such cells. More particularly, it relates to a positive terminal construction for a lithium/lithiun halide/iron sulfide cell.
Lithium/lithium halide/iron sulfide cells, which are frequently simply referred to as lithium-iron sulfide cells, are well known in the prior art. Such cells typically employ metallic lithium as the reactant in negative electrodes. The reactant of the positive electrode is generally iron sulfide or iron disulfide. A suitable molten salt electrolyte maintained at a temperature in excess of 400.degree. C is employed to provide the necessary ionic conduction between the electrodes.
The components of a lithium-iron sulfide cell are usually assembled within a stainless steel outer housing. Electrical connection is generally provided between the housing and the negative electrode so that the housing serves as the negative terminal of the cell.
One of the major problems encountered in constructing a lithium-iron sulfide cell involves the design of the positive terminal for the cell. The prior art practice is to use a metallic rod to connect the positive electrode to the external circuit. The metal chosen must be a good conductor of electricity and inert to chemical attack by the cell environment. Typically tungston or molybdenum is chosen.
In order to connect the positive electrode to the external circuit, the metallic terminal rod extends through the outer housing. Passing the rod through the housing, which also serves as the negative terminal, presents several major problems. The positive terminal rod must be insulated from the housing. The insulator material must be carefully chosen to perform its function under the operating conditions of the cell and particularly at the operating temperature of the cell. Further, the insulating material must be inert to chemical reaction with the components of the cell and particularly with the positive electrode reactant. Another problem involved in passing the positive terminal through the outer housing is that the cell must be constructed airtight. Air reacts with the reactants in the electrodes, reducing their effectiveness in the cell reaction. Leakage of air into the cell will shorten the cell's lifetime and reduce its current output. Creating a hermetic seal is complicated because of the elevated operating temperatures of the cell. During its lifetime, the cell is frequently cycled between ambient temperatures and about 450.degree. C. Because such a large temperature change is involved, the thermal coefficients of expansion of the materials employed to seal the positive terminal and the outer housing are a critical factor to be considered. Even small differences in the expansion coefficient of the materials as compared to the housing and positive terminal will cause the seal to fail as the temperature changes.
In a lithium-iron sulfide cell, the construction which includes the positive terminal rod and insulates it from the housing, while sealing it thereto, is commonly termed a feedthrough assembly. A prior art feedthrough assembly now commonly used employs a mechanical seal. There a ceramic insulator sheath is closely fitted around the terminal rod. The rod and sheath are passed through the male portion of a compression fitting which has been welded to the cell housing. A stainless steel compression nut is then turned down about the threads of the male portion and the seal is effected. The chief disadvantage in using such a mechanical seal is that it does not remain airtight. Some investigators have proposed that the terminal rod, the insulator and the outer housing can be joined together by brazing. However, known brazing materials react with the constituents of the cell, thus reducing active cell material and destroying the seal.
Therefore, in view of the problems described above, it is an object of the present invention to provide an improved construction of a feedthrough assembly for an electrochemical cell such as a lithium-iron sulfide cell. It is a more specific object of the present invention to provide a feedthrough assembly having an insulator body formed of aluminum nitride, yittria or mixtures thereof and having molybdenum connectors embedded therein for sealing the insulator to the cell housing and to the terminal rod. This construction permits a durable, hermetic seal to be formed between the terminal and the housing via the molybdenum connectors. These and other objects of the invention will become more readily apparent from the following description.